Hydraulic systems frequently employ a hydraulic motor to raise and lower relatively heavy loads and at times to support such loads in an elevated position. When the motor is required to support the load in such elevated position, it is normally desirable to isolate the relatively high load generated pressure in the load supporting end of the motor from the remainder of the system. This is to prevent the downward drifting of the load due to leakage past the valve spools of the conventional control valves normally used in such systems. The load pressure is also normally isolated for safety purposes to prevent the sudden dropping of the load in the event of a line failure or the like.
This isolation is normally accomplished by the disposition of a load check valve in the motor line near or preferably at the load supporting end of the motor. Such load check valve permits free flow of fluid to the motor, but normally prevents the escape of fluid therefrom. Thus when it is desired to lower the load, it is necessary to permit the load check valve to open so that fluid may be discharged from the load supporting end of the motor. For a vented-type check valve popularly used, this is accomplished, as more fully described in U.S. Pat. No. 3,127,688 to Hein et al, by simultaneously venting the pressure holding the load check valve closed when the control valve is actuated to its load lowering position.
While the use of load check valves is normally essential, they have in the past had many adverse effects on the load lifting systems in which they are employed. One problem commonly associated with their use is hydraulic hammering. It will be appreciated that because the load check valve is so effective in isolating the pressure at the load supporting end of the motor, the pressure thereat is sometimes extremely high due to a heavy load, whereas the pressure on the control valve side of the load check valve is relatively low. As a result, when the load check valve pops open as is normally the case, a tremendous pressure surge occurs towards the control valve causing such hammering. The severity of the hammering is frequently such to cause a loud audible bang throughout the system. In addition, the pressure waves created thereby tend to reverberate causing an undesirable fluttering of the load check valve which in turn makes the lowering of the load erratic and difficult to control. Also associated with the quick release of the high pressure from the motor is the relatively small but sudden drop of the load before the balance of pressure occurs.
Another problem commonly associated with the use of load check valves is that when a heavy load is being lowered and the control valve is shifted to neutral to stop such lowering, the load check valves will normally pop closed preventing further escape of fluid from the motor. When this happens, the weight and momentum of the load causes an extremely high pressure spike to occur in the motor which can be injurious to the motor and its various components.
In earthmoving vehicles, such as a hydraulic excavator and the like, where two or more hydraulic motors are connected in unison for raising and lowering the load, it is desirable that the pressures in each of the motors be equal so as to prevent uneven operation and the cocking of the excavator's boom. As the load check valve normally prevents this pressure equalization, a separate small equalizer line has been used in the past which bypasses the load check valves so as to permit such pressure equalization. However, the addition of such equalizer line not only adds to the complexity of the system, but also adds another line which is susceptible to breakage or rupture during operation to cause the disablement of the system.